The present invention relates to a lens shape measuring instrument for measuring data required for grinding the end surface of an eyeglass lens, particularly the thickness and thickness center of the end surface of an eyeglass lens.
In order to engage an eyeglass lens correctly and properly into an eyeglass frame, the eyeglass lens must be ground in such manner that the peripheral surface of the eyeglass lens fits a groove on the eyeglass frame and that the planar shape of the eyeglass lens fits the planar shape of the eyeglass frame. Also, the convex projection (convex projection on the periphery of the eyeglass lens) must be formed on the end surface of the lens so as to fit the groove of the eyeglass frame.
The present applicant has proposed an automatic lens grinder for grinding the peripheral surface of an eyeglass lens so as to form a convex projection which accurately fits the groove of an eyeglass frame and has disclosed same in Japanese Patent Publication No. 58-43227 and Japanese Patent Application No. 60-97295.
In the invention of said Japanese Patent Publication No. 58-43227 and Japanese Patent Application No. 60-97295, an eyeglass lens is movably supported in the direction of an optical axis, and the end surface of the eyeglass lens is brought into contact with a grindstone having a V-groove. The end surface of the eyeglass lens is moved along the V-groove when the eyeglass lens is rotated by one turn by rotating the eyeglass lens grindstone, and the thickness center of the end surface of the eyeglass lens is measured based on the movement of the lens in the direction of its optical axis. The distance between the end surface of the eyeglass lens and the optical center of the lens is measured by means of the movement of the lens in a vertical direction.
According to the conventional method, to measure the thickness and thickness center of the end surface of an eyeglass lens, contactors are attached on the front surface and rear surface of the eyeglass lens, and the front surface and rear surface of the contactors are detected, and the distance between the two detected positions is detected by a distance detector such as an encoder.
With rapid propagation of very thin plastic lenses in recent years, there are many lenses which have narrower cutting allowance and which are made of soft lens material. In this respect, according to the invention of Japanese Patent Publication No. 58-43227 and Japanese Patent Application No. 60-97295, cutting allowance may be scraped off before the width center of the grindstone corresponds with the thickness center of the lens end surface. If contact pressure on the grindstone is reduced to decrease the amount cut from the lens, smooth sliding in the direction of inclination of the grindstone does not occur between the lens and the inclined surface depending upon each condition, and the lens ascends the inclined surface. As a result, the lens cannot follow the center of the V-groove, and the thickness center of the lens end surface may not be measured.
In case the contactor is attached on the front surface and rear surface of the eyeglass lens as described above, the surface of the eyeglass lens may be damaged. Also, complicated mechanisms such as sensor installing and moving mechanism or distance measuring mechanism are required, thereby resulting in higher cost.